1. Field of the Invention
The present invention relates to a power conversion device capable of continuous operation of an AC motor in stable fashion even when there is a power cut (that is to say, blackout, outage, power failure or power interruption) of the AC power source for a short time or an instantaneous voltage drop (hereinbelow these will be referred to by the general term of instantaneous power cuts).
2. Description of the Related Art
AC motors typified by induction motors are now frequently driven by power conversion devices such as inverter devices. It has become common that, when an instantaneous power cut has occurred during operation of the inverter device, once operation of the inverter device has been stopped, the inverter device is restarted after restoration of power and operation of the AC motor is then continued by reconnecting the motor. However, in this case, the AC motor is free running during the instantaneous power cut, so its speed falls considerably. Also, in order to restart the motor, processing is required to again raise the voltage/frequency ratio from the vicinity of zero, so restarting takes some time, which is undesirable in terms of the load driven by the AC motor.
In this connection, in the case of an instantaneous power cut, the method is also available of restarting without stopping the output of the inverter device. In this case, if power continues to be supplied by the inverter device even after occurrence of the instantaneous power cut, the DC voltage rapidly decreases. There was therefore the risk of excess rush current due to recharging of the smoothing capacitor when power was restored in a condition in which the DC voltage had been decreased in this way. A further concern was overcharging of the smoothing capacitor due to resonance with the system impedance.
A countermeasure in respect of the above is a method wherein the output of the inverter device is not stopped when an instantaneous power cut occurs, but the AC motor is operated in a regenerative condition with the frequency level and the voltage level that are supplied thereto lowered at a prescribed rate, the frequency level and the voltage level being restored at a prescribed rate after power restoration. An example is to be found in issued Japanese Patent No. 2543664 “Method of controlling an AC motor”.
A further technique that has been proposed is to control the frequency and voltage of the inverter device such that the torque current instruction becomes practically (substantially) zero when an instantaneous power cut occurs. An example is to be found in published Japanese Patent No. H. 7-20394 “Power conversion device”.
With the technique disclosed in the former, the drop in DC voltage can be suppressed. However, there was the problem that, since the motor was deliberately controlled into the regenerative operation region, a greater drop in speed of the AC motor was experienced during the instantaneous power cut (blackout, outage or power failure) period than would have occurred in natural deceleration. A further problem was that time was required to raise the voltage in stable fashion to the normal operating condition after the voltage had been restricted in this way.
Also, with the technique disclosed in the latter, although the dropping in DC voltage can be suppressed so control during instantaneous power cut (blackout, outage or power failure) is comparatively easy, the inverter continues to supply power to the motor until the voltage falls below the level of the AC voltage drop, and, in addition, even if control of the torque current of the inverter is commenced as soon as the voltage falls below the level of the AC voltage drop, since there is a delay in response until the torque current is controlled to practically zero, the suppression of the reduction of the DC voltage was insufficient.
Accordingly, one object of the present invention is to provide a novel power conversion device wherein the reduction in the DC voltage during the period of an instantaneous power cut can be further suppressed and, after power recovery, restoration of the motor to the normal operating condition can be effected in a stable and rapid fashion.